The present invention relates to the aqueous emulsion polymerization of fluorinated monomers to produce fluoropolymers. In particular, the present invention relates to the aqueous emulsion polymerization involving liquid fluorinated monomers which have a boiling point of at least 50xc2x0 C.
Aqueous emulsion polymerization is a well-known method for making fluoropolymers, i.e. polymers of which the carbons in the backbone of the polymer have fluorine atoms attached to them. Generally, the amount of fluorination of the backbone of fluoropolymers will be at least 40% by weight, preferably at least 50% by weight. Fluoropolymers which have fully fluorinated backbones are called perfluoropolymers.
The fluoropolymers are generally produced by (co)polymerization of gaseous fluorinated monomers, in particular fluorinated olefins such as tetrafluoroethylene (TFE) hexafluoropropylene (HFP), vinyl fluoride, vinylidene fluoride (VDF) or chloro-trifluoroethylene (CTFE).
To obtain certain desired properties, it is known to copolymerise the aforementioned gaseous fluorinated olefins with monomers such as fluoro alkenyl ethers which include fluoro vinyl ethers and fluoro allyl ethers. Fluoro alkenyl ethers that have a high boiling point of 50xc2x0 C. or more have a low vapor pressure at typical polymerization temperatures of 20xc2x0 C. to 100xc2x0 C. Additionally, they have an extremely low water solubility. As a result it has proven difficult to incorporate these liquid comonomers into fluoropolymers at larger levels as may be desired for certain applications. Incorporating large amounts of such comonomers is of particular value for producing elastomers that have a low Tg. When one wants to improve the amount incorporated of such monomers by increasing the polymerization temperature, that also increases the likelihood of undesired chain transfer reactions which impair the possibility of obtaining high molecular weight. On the other hand, polymerization at low temperature reduces the rate of polymerization making the process economically less attractive.
EP 219065 discloses an aqueous free radical emulsion polymerization of perfluoro vinyl ethers of the formula CF2xe2x95x90CFxe2x80x94Oxe2x80x94(CF2CFXO)mxe2x80x94Rf wherein X is F or CF3, m is 1 to 5 and Rf is a C1-C6 perfluoroalkyl group. EP 219065 discloses the use of carboxylate terminated perfluoropolyether emulsifiers for conducting the polymerization. However, in the examples of this EP-patent, fairly large amounts of the emulsifier are used, typically about 10% by weight based on the aqueous phase. Such high emulsifier concentrations usually lead to a so-called micro-emulsion polymerization which typically results in polymer particles of considerably less than 100 nm.
EP 250767 discloses an aqueous free radical emulsion polymerization involving for example perfluoroalkyl perfluorovinyl ether monomers by using a microemulsion of a perfluoropolyether with polyether based emulsifiers. This microemulsion is taught to have droplet sizes of less than 200 nm. The microemulsion is added to the polymerization medium at the beginning of the polymerization. The resulting particle sizes of the polymer particles obtained is very small, e.g. 40 nm, which is evidence that the polymerization occurs via microemulsion polymerization. Thus, the number of polymer particles per liter is increased by an order of magnitude. Therefore, the polymerization rate is considerably increased. However, EP 250767 does not describe pre-emulsification of liquid polymerizable fluorinated monomers like perfluoroalkyl perfluorovinyl ether monomers.
EP 816397 discloses the use of similar microemulsions of perfluoropolyethers with polyether based emulsifiers for the aqueous emulsion polymerization of vinylidene fluoride (VDF). As disclosed, the obtained latex particles have a very small size (apparently well below 100 nm) and likewise an increase of the polymerization rate is found. The polymerization here therefore evidently also occurs via microemulsion polymerization.
U.S. Pat. No. 4,864,006 also discloses the use of perfluoropolyethers microemulsified in perfluoroether-based emulsifiers at the aqueous emulsion polymerization of fluorinated monomers. All of these processes lead to microemulsion polymerizations characterized by an increased polymerization rate due to the observed small particle size, considerably lower than 100 nm.
The perfluoropolyether emulsifiers have the disadvantage that they have high boiling points and are difficult to remove from the polymer resin. Residual emulsifiers can have an adverse effect on the processing properties and the properties of the final product, for example lead to discoloration during work-up or processing, or to bleed-out during long-term use of the final product.
U.S. Pat. No. 5,608,022 teaches the aqueous emulsion polymerization of sulfonyl fluoride or carboxylic acid ester containing fluorinated vinyl ethers wherein the so-called functionalized vinyl ether monomer is pre-emulsified. According to Example 1 of this US-patent, the droplet size of the monomer droplets in the emulsion is 550 nm.
It would be desirable to find an improved aqueous emulsion polymerization process that allows for effective incorporation of large amounts of highly boiling fluorinated monomers. Desirably, the polymerization reaction should proceed at a high speed allowing for a more cost effective production of such fluoropolymers. The process desirably may be run using low emulsifier levels and using emulsifiers that can be readily removed from the resulting polymerization product.
The present invention provides a method of preparing a fluorinated polymer including repeating units derived from a liquid fluorinated monomer having a boiling point of at least 50xc2x0 C. and selected from the group consisting of fluorinated olefins, fluorinated allyl ethers and fluorinated vinyl ethers that do not contain hydrolysable groups that upon hydrolysation yield ionic groups, the method comprising the steps of:
pre-emulsifying said liquid fluorinated monomer in water with the aid of a fluorinated emulsifier to obtain an aqueous emulsion of said fluorinated monomer; and
polymerizing a thus obtained emulsified fluorinated monomer.
Preferably the pre-emulsified liquid fluorinated monomer is copolymerized with one or more gaseous fluorinated monomers.
By the term xe2x80x9cliquid fluorinated monomerxe2x80x9d is meant that the monomer is generally present as a liquid at ambient conditions of temperature and pressure, i.e. at a temperature of 20xc2x0 C. and a pressure of 1 atm. By the term xe2x80x9cpre-emulsifiedxe2x80x9d in connection with the present invention is meant that the fluorinated monomer is emulsified in water with the aid of the fluorinated emulsifier prior to polymerization of the liquid fluorinated monomer. The term aqueous emulsion is to be understood as a liquid emulsified in water that generally has a milky appearance generally having a settling time of at least 1 hour. Such settling time is generally achieved using a fluorinated emulsifier (preferably a non-telogenic emulsifier) other than a fluorinated polyether emulsifier. By the term xe2x80x9cboiling pointxe2x80x9d in connection with the present invention is meant a boiling point under ambient conditions, i.e. at a pressure of about 1 atm. By the term xe2x80x9cgaseousxe2x80x9d in connection with the present invention is meant that the respective compounds are present as a gas under ambient conditions of temperature and pressure, i.e. at a temperature of about 20xc2x0 C. and a pressure of about 1 atm. The fluorinated vinyl ether should be free of hydrolysable groups that upon hydrolysation yield ionic groups, in particular the fluorinated vinyl ether should not contain hydrolysable groups such as esters and SO2F. The fluorinated vinyl ether may contain substituents like chlorine, bromine and iodine, fluorinated alkyl groups, fluorinated alkoxy groups and fluorinated polyether functions.
It was found that when the liquid fluorinated monomer was emulsified prior to its copolymerization with gaseous fluorinated monomers, the polymerization time could be reduced and high levels of the liquid fluorinated monomer could be readily copolymerized. In particular, the process of the invention allows for the production of fluoropolymers having at least 1% by weight of repeating units derived from the pre-emulsified fluorinated monomer, preferably at least 5% by weight and more preferably at least 10% by weight. The process of the invention also makes it possible to produce homopolymers of the pre-emulsified liquid fluorinated monomer. Also, commonly employed fluorinated emulsifiers that can be readily removed subsequent to polymerization can be used and they can be used in fairly low levels, typically of about 0.1% by weight to 1% by weight relative to the amount of water.
In accordance with the process of the present invention, the fluorinated monomer is emulsified in water with the aid of a fluorinated emulsifier prior to its polymerization or copolymerization with the gaseous fluorinated monomers. The pre-emulsification of the liquid fluorinated monomer preferably results in an emulsion having monomer droplets of an average diameter of 1 xcexcm or more, typically it is expected in the range of 1 to 20 xcexcm. The aqueous emulsion should preferably have a pot life (settling time) of at least 1 hour, more preferably at least 3 hours. The pot life or settling time is defined as the time required for 10% by weight of the monomer droplets to settle or separate out of the aqueous emulsion.
Aqueous emulsions of the liquid fluorinated monomer can conveniently be obtained by suitable emulsification equipment such as for example a dispersing device such as Ultra-Turrax. The stirring rates should be sufficiently high to achieve the desired degree of emulsification and stability. Generally, stirring rates of 24 000 rpm or more can be employed. According to indication of the supplier of the Ultra-Turrax device, the resulting emulsions would have an average droplet size of 1 to 10 xcexcm. Air is preferably excluded during the emulsification.
Fluorinated emulsifiers for preparing the aqueous emulsion of the liquid fluorinated monomer are preferably non-telogenic emulsifiers. Examples of fluorinated emulsifiers include salts, in particular ammonium salts of linear or branched perfluoro alkyl carboxylic and sulphonic acids having 6 to 11 carbon atoms in the alkyl chain. It was found that salts of branched perfluoroalkyl carboxylic and sulphonic acids are more effective than their linear counter parts. Specific examples include perfluorooctanoic acid ammonium salt (APFOA) and C8F17SO3Li. Commercially available fluorinated emulsifiers that can be used include C8F17SO3Li available as FT 208 from Bayer AG, and C8F17SO2N(C2H5)CH2COOK available from 3M as FC 128.
APFOA is the preferred emulsifier as it can be more readily removed from the polymerization product at the end of polymerization. Typically, APFOA is very volatile and decomposes at temperatures of 180xc2x0 C. or more leading to volatile products. Therefore, APFOA type of emulsifiers can be easily removed during work-up of the fluoropolymers.
The pot life of the aqueous emulsion of the liquid fluorinated monomer can be increased by increasing the pH. For example, with APFOA, the pot life can be increased to 50 hours or more at a pH of 7 or more. Typically, the pH of the aqueous emulsion can be increased by adding ammonia or alkali metal hydroxides.
The amount of fluorinated emulsifier used to emulsify the liquid fluorinated monomer is generally between 0.1 and 1% by weight based on the weight of the water phase. Although higher amounts of emulsifier can be used, they will not necessarily lead to a significant increased pot life of the aqueous emulsion.
The aqueous emulsion of the liquid fluorinated monomer may include a mixture of liquid fluorinated monomers and may further comprise other monomers such as other poorly water soluble and liquid monomers. Additional emulsifier may be added and if added, they are preferably identical to the emulsifier(s) used to pre-emulsify the liquid fluorinated monomer. Advantageously, APFOA is used in the emulsification of the liquid fluorinated monomer and in the subsequent aqueous emulsion polymerization.
The pre-emulsification of the liquid fluorinated monomer may proceed in a separate vessel from the polymerization vessel, in the polymerization vessel itself prior to commencing the polymerization or in the monomer supply line to the polymerization vessel.
The aqueous emulsion polymerization can be carried out continuously in which, for example, the aqueous emulsion of the liquid fluorinated monomer and other monomers, water, optionally further emulsifiers, buffers and catalysts are fed continuously to a stirred reactor under optimum pressure and temperature conditions while the resulting emulsion or suspension is removed continuously. An alternative technique is batch or semibatch polymerization by feeding the ingredients into a stirred reactor and allowing them to react at a set temperature for a specified length of time or by charging ingredients into the reactor and feeding the monomers and emulsified liquid fluorinated monomer into the reactor to maintain a constant pressure until a desired amount of polymer is formed. The polymerization can be carried out in a standard or conventional vessel used for emulsion polymerization of gaseous fluorinated monomers.
For the free-radical polymerization use may be made of any suitable initiator or any suitable initiator system, for example ammonium persulfate (APS), or of redox systems, such as APS/bisulfite and potassium permanganate. If oil-soluble initiators are used in the polymerization, it is generally preferred for these to be mixed with the aqueous emulsion of the liquid fluorinated monomer. For the purposes of the present invention, oil-soluble initiators are those which have no, or only insufficient, solubility in water. Examples of oil-soluble initiators are substituted dibenzoyl peroxides and cumene hydroperoxides, in particular bisperfluoropropionyl peroxide.
The polymerization systems may comprise auxiliaries, such as buffers and, if desired, emulsifiers, complex-formers or chain-transfer agents. The polymerization temperature may be from 10 to 100xc2x0 C. Polymerization pressures may be from 3 to 30 bar.
The achievable solids content of the polymer dispersion is typically from 10 to 35% without any major coagulation. Surprisingly, the resultant dispersions are highly stable despite the observed unusually large particle diameters of up to 400 nm. The particle size of polytetrafluoroethylene (PTFE) born via standard emulsion polymerization do not exceed 250 nm, larger particle size result in coagulation of the PTFE at polymerization conditions; the particle size of fluoroelastomers and thermoplasts born in standard emulsion polymerization usually do not exceed 200 nm. The large particle size observed is believed to be due to the emulsion polymerization occurring partially via a so-called monomer-droplet polymerization.
The production of fluoropolymers via aqueous emulsion polymerization generally produces considerable amounts of industrial wastewater as a consequence of necessary washing processes during the isolation of the resins from the polymer dispersion. The wastewater generally comprises about 90% of the emulsifier used. Since the emulsifier makes a considerable contribution to production costs, it is preferably recovered and recycled from the wastewater, using a process as disclosed in German Patent Application 199 53 285.0.
The aqueous emulsion polymerization process of the present invention can be used for the polymerization of liquid fluorinated monomers that have a boiling point of at least 50xc2x0 C. and that are selected from the group consisting of fluorinated olefins, fluorinated allyl ethers and fluorinated vinyl ethers that do not contain hydrolysable groups capable of yielding ionic groups when hydrolysed. The liquid fluorinated monomer may be fully or partially fluorinated, i.e. one or more fluorine atoms may be replaced by iodine, bromine, chlorine or hydrogen. However, when the liquid fluorinated monomer is partially fluorinated, the molar ratio of fluorine to non-fluorine atoms or groups should generally be at least 4. Examples of liquid fluoroolefins include partially and fully fluorinated olefins having a boiling point of at least 50xc2x0 C. Specific examples include perfluoroalkyl vinyl monomers having 6 or more carbons in the perfluoroalkyl chain such as C6F13xe2x80x94CHxe2x95x90CH2.
The process of the invention may further be used to polymerize liquid fluorinated allyl ethers which are fluorine compounds that have at least one ether bridge and one terminal double bond, with the oxygen in xcex2-position with respect to the double bond. Examples of fluorinated allyl ethers include those that are fully as well as those that are partially fluorinated. Examples of fluorinated allyl ethers are those that correspond to the formula:
Rfxe2x80x94Oxe2x80x94CF2xe2x80x94CFxe2x95x90CF2xe2x80x83xe2x80x83(I)
wherein Rf represents a linear or branched perfluoroalkyl group which may contain one or more ether linkages. A specific example of a liquid fluorinated allyl ether according to formula (I) is CF3xe2x80x94Oxe2x80x94(CF2)2xe2x80x94Oxe2x80x94CF2xe2x80x94CFxe2x95x90CF2 (boiling point 67xc2x0 C.).
Preferably, the liquid fluorinated monomer for use in the invention is a fluorinated vinyl ether. Generally, the fluorinated vinyl ether is a perfluoro vinyl ether. Typically, the fluorinated vinyl ether is a perfluorinated vinyl ether corresponding to formula (II):
CF2xe2x95x90CFO(RfO)n(Rxe2x80x2fO)mRxe2x80x3fxe2x80x83xe2x80x83(II)
wherein Rf and Rxe2x80x2f are different linear or branched perfluoroalkylene groups of 2-6 carbon atoms, m and n are independently 0-10 and the sum of n and m is at least 1, and Rxe2x80x3f is a perfluoroalkyl group of 1-6 carbon atoms.
Examples of fluoro vinyl ethers of formula (II) include compounds of the formula (III):
CF2xe2x95x90CFO[CF2CF2CFZO]nRxe2x80x3fxe2x80x83xe2x80x83(III)
where Rxe2x80x3f is a perfluoroalkyl group having 1-6 carbon atoms, n=1-5, and Z=F or CF3. Examples of compounds of formula (III) include those in which Rxe2x80x3f is C3F7, n=1, Z=F or CF3 and CF3xe2x80x94Oxe2x80x94CF(CF3)xe2x80x94CF2xe2x80x94CF2xe2x80x94Oxe2x80x94CFxe2x95x90CF2 (boiling point 85xc2x0 C.). Further useful fluorinated vinyl ethers included in formula (II) correspond to the formula (IV):
CF2xe2x95x90CFxe2x80x94Oxe2x80x94(CF2CFXO)mxe2x80x94Rfxe2x80x83xe2x80x83(IV)
in which m represents an integer of 1 to 5, X is F or CF3 and Rf is a C1-C5 perfluoroalkyl group. Examples of monomers of formula (IV) include those where X is CF3 and Rf is perfluoro n-propyl and wherein m is 1 (boiling point of 103xc2x0 C.) or m is 2 (boiling point of 160xc2x0 C.).
Additional perfluoro vinyl ether monomers useful in the invention include compounds of the formula
CF2xe2x95x90CFO[(CF2CFCF3O)n(CF2CF2CF2O)m(CF2)p]CxF2x+1xe2x80x83xe2x80x83(V)
where m and n=1-10, p=0-3, and x=1-5. Preferred members of this class include compounds where n=1, m=1, and x=1.
Still further examples of perfluoro vinyl ethers include ethers corresponding to formula (VI):
CF2xe2x95x90CFOCF2CF(CF3)xe2x80x94Oxe2x80x94(CF2O)mCnF2n+1xe2x80x83xe2x80x83(VI)
where n=1-5, m=1-3, and where, preferably, n=1.
Copolymerization of perfluorinated vinyl ether monomers such as set forth above with gaseous fluorinated monomers such as TFE, results in polymers with considerably improved properties. When incorporated at low levels, typically less than 10% by weight, fluorinated thermoplastics with improved properties can be obtained. Fluoroelastomers with desirable properties can be produced by copolymerizing substantial amounts of fluorovinyl ethers with gaseous fluorinated monomers. In particular the low-temperature properties of the final product can be improved by incorporating high levels of the liquid fluorinated vinyl ether monomer into the fluoropolymer. Typically, the amount of repeating units derived from such fluorovinyl ethers, for example those according to formula (II) above, will be between 15 mol % and 50 mol %. Such fluoropolymers will have a glass transition temperature (Tg) that is, for example, as low as xe2x88x9240xc2x0 C.
Especially useful fluoro vinyl ethers for making fluoroelastomers with a low Tg are those that have linear perfluoro groups attached to the vinyl of the fluoro vinyl ether, such as monomers according to formula (III) in which X is F. A highly preferred monomer for making fluoroelastomer is CF3xe2x80x94Oxe2x80x94(CF2)3xe2x80x94Oxe2x80x94CFxe2x95x90CF2 (boiling point of 64xc2x0 C.).
The preparation of vinyl/allyl ethers is generally expensive and has been described in DE-A-22 15 401. The process of the invention allows for the effective incorporation of these monomers into fluoropolymers without substantial waste of these expensive monomers.
To manufacture fluoropolymers and preferably perfluoropolymers by the process of the present invention, the liquid fluorinated monomers are generally copolymerized with gaseous fluorinated monomers, in particular gaseous fluorinated olefins. Examples of gaseous fluorinated monomers for copolymerization with the liquid fluorinated monomers include tetrafluoroethylene (TFE) hexafluoropropylene (HFP), vinyl fluoride, vinylidene fluoride (VDF), chlorotrifluoroethylene (CTFE) and mixtures thereof. Additionally, the copolymerization may involve non-fluorinated gaseous monomers such as ethylene and propylene.
Generally, the amount of repeating units derived from the gaseous fluorinated monomers in the fluoropolymer will be at least 20% by weight, preferably at least 40% by weight. A typical amount of repeating units derived from gaseous fluorinated monomers is between 50% by weight and 95% by weight.
Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of the present invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.
The invention will now be further illustrated with reference to the following examples without however intending to limit the invention thereoto.